Detractors warn she could become a Ralph Nader-like candidate, taking enough votes from Clinton to deliver Republican nominee Donald Trump a victory in November

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Jill Stein, presumptive 2016 Green Party presidential nominee, speaks at a Bernie or Bust rally during the Democratic National Convention in Philadelphia, on Tuesday, July 26, 2016.

Angry and disaffected Bernie Sanders' backers have a new rallying cry: "Jill not Hill."

That's Green Party candidate Jill Stein, whose liberal agenda of tuition-free college, $15-per-hour minimum wage and a renewable energy economy by 2030 offers a home to Sanders' supporters disillusioned by the two-party political system and unwilling to back Democrat Hillary Clinton.

"We are standing up together, we are supporting the Bernie delegates who liberated themselves tonight from the Democratic Party," Stein belted through a megaphone to cheering supporters outside the Democratic convention Tuesday night.

Stein is a 66-year-old doctor and political activist from Massachusetts who, like Clinton, was born in Illinois and raised in a Chicago suburb. She is poised to become the Green Party's 2016 presidential nominee early next month, a title she won in 2012.

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Then, Stein failed to crack even half a million votes. This year, detractors warn she could become a Ralph Nader-like candidate, taking enough votes from Clinton to deliver Republican nominee Donald Trump a victory in November.

Nader, the Green Party's nominee in 2000, captured nearly 3 million votes. Many Democrats argue it was Nader who kept Democrat Al Gore from winning the White House over Republican George W. Bush.

"If you're in a battleground state," he said. "You've got to suck it up and vote for Hillary."

Clinton's campaign is monitoring many of Sanders supporters who, for now, seem unwilling to get behind her as the nominee despite the senator's pleas.

With voters expressing discontent at both major party nominees, pollsters are watching third-party candidates like Stein and Gary Johnson, the Libertarian party's nominee. But a July Associated Press-GfK poll found both Stein and Johnson remain virtual unknowns among Americans, with 76 percent saying they don't know enough about Johnson to have a favorable or unfavorable opinion and 82 percent saying the same about Stein.

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Stein first ran for elected office with the Green Party in 2002, challenging Mitt Romney for the Massachusetts' governorship but winning just a fraction of the votes. Stein, who graduated from Harvard Medical School in 1979, promises to wipe out student debt entirely and favors universal health care provided by a single public entity.

"I've been working my butt off for Bernie for a year, but I know a good deal about Jill Stein," said Stacy Bucek of Winter Park, Florida. "A lot of my beliefs line up with how she stands."

Stein is working aggressively to win over Sanders supporters, appearing at rallies in downtown Philadelphia during the week of the convention as protesting delegates shout "Jill not Hill!" And she's taking the fight directly to Clinton, accusing the Democratic Party of engaging in an "intimidation campaign that is trying to get you to vote for some lesser evil."

"Hillary cannot stand up to Donald Trump because it's the policies of the Clintons that have created the economic misery that has given rise to the right-wing extremism that supports Donald Trump," Stein said.

Danny Keating, a steelworker from Lowell, Massachusetts, says he'll vote for Stein "because the two-party system is corrupt, it's becoming more evident by the rigged primaries and everything else, I'm here to fight for a party for the 99% and working people."